As a general note, the Heath hen's main characteristics were a reddish hue and darker overall. Broader barring of the chest, ventral area and side pockets , pointed to narrowly rounded pinnae, and olive-brown to pale clove brown tail.
During the transit from Europe to Texas, the plank of wood he was attached to came loose from the base. That opportunity was used to remove him and take as many comparative pictures as possible.
Here is a picture of the whole sides of the birds, note that the bird on the right is a greater from a nice central location of their range like Nebraska, the airsacks/eyecombs were painted yellow to be a representation of the critically endangered Attwater's Prairie chicken.
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A comparison picture of the Heath hen (Left) and Greater's (Right) flanks.
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Probably my favorite picture, showing the crop areas of both birds. A very distinguishing attribute that the Heath Hen shows is a heavy reddish wash on the crop area.
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The Greater/Lesser/Attwater's have an almost black tail, some having a lighter highlight on the leading edge. The Heath Hen's tail was much different as can be seen.
According to Ridgway & Friedmann (1946), tail of the Heath hen was olive-brown to pale clove brown.
[You can also see in the Heath hen's tail remains its true color because the dark black parts of the rump feathers are still in original color]
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This is a nice picture showing the front views of the Heath Hen (Left) and Greater (Right). The very thick barring and overall rusty hue of the Heath Hen is apparant.
Referring again to Ridgway & Friedmann (1946), General tone of the upper parts averages less rufescent, the broad tips of the feathers of the back, rump, and upper tail coverts especially are less rufescent than in the Heath Hen (being pinkish buff) instead of the pale tawny-olive found in the Greater Prairie Chicken.
A closer aspect of the front view, amazing the amount of difference.
A top view with all three birds in it (Heath Hen far right) shows the rufescent color, with the two Greaters to show the distinctive difference.
Literature on descriptions is very abstract on certain characteristics. For example Ridgway and Friedmann describe the Greater prairie chicken as "having no conspicuous buffy-whitish terminal spots on the scapulars". They however do in reality, normally not as broad but can be as broad if not broader than the Heath Hen.
A species key from James Greenway Jr. 1967 lists the Heath Hen as having "axillaries usually banded or heavily spotted". Ridgway and Friedmann list the Heath Hen as having "under wing coverts barred white and buffy brown on the outer ones, the inner ones and the axillars almost wholly white".
[The axillaries being the feather group located on the underwing between the wing feathers and the body]
This view shows a closer comparative for the Greater (Left) Heath Hen (Right) and the color differences in more detail.
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From the side of all three birds, a nice picture showing the smaller size, reddish color, and darkness of the bird.
Here is a closeup of the pinnae, showing heavy reddish hues. Even the black feathers of the pinnae have a reddish hue instead of black.

The barring is supposed to be around 5-6mm (about 50% more than the Greater) A feather sample was taken from the Heath Hen and measured the outer bar that was 6mm wide, and one from the Greater (as close to the same spot as humanly possible) that was 4mm. You can see the thicker barring and reddish color showing up again. The feather from the Heath Hen was taken on the opposite side to ensure a definitive nonfaded sample.
This picture shows the two feathers separated. The subterminal white bar is essentially the same but differs in the terminal white ridge of the feather being thinner in the Heath Hen and thicker reddish bars in both the upper and lower portion of the feather.
Recent genetic studies have shown that the Heath Hen was most closely related to the Greater Prairie Chicken from Wisconsin. Along with wondering what affect aging plays, I found a mounted specimen of a Greater from the Wisconsin area dated from the early part of the 1900's. This a comparative photo of the crop areas of my Heath Hen compared to this bird.

Note the strong reddish hue of the Heath hen on the right, and lack thereof on the old Greater on the left.
Here is a photo of a Greater prairie chicken from the early 1900's. It is important to use this to level the playing field in a manner since all Heath Hen mounts are around the same age.

The first area shows the crop lacking the reddish-wash.
Second area showing lack of prominant buffy terminal ends to the scapulars (Note: All Heath Hens do not have this greatly distinct feature, and have viewed Greaters with just as prominate markings if not more).
Third area showing a darker tail than is typically seen on a Heath Hen. Most of the difference comes as you go away from the center feathers where the distinction is most prominant.
Overall the bird has barring thinner than found in Heath Hens. Also noticable is the elongated beak, with a more pronounced hook at the end.
FINAL NOTE: My Heath hen is very much sterotypical in regards to common characteristics. The point is not to compare a 100 year-old mount to a newer one, but to illustrate the differences indicitive of the Heath Hen in comparison to the Greater Prairie Chicken.
It is a given that a victorian era bird had an infinite amount of variables that could alter certain colors on the bird. So that is a prime reason to include a couple of greater prairie chickens from the same era to use in comparison. Even though some changes may have occured, none have prompted a situation where a greater will turn into a Heath hen over time or vice versa.